1. Field of the Invention
The present invention relates to a traffic sensor having bipolar or multi-polar piezoelectric sensing elements which uniquely identify a lane in which a vehicle is detected, and more particularly, to a piezoelectric cable or film which when stretched across a roadway generates electrical signals of different polarities or of different states in respective lanes of the roadway so that the lane from which one or more electrical signals are received may be readily discriminated by the polarity or state of the received electrical signal(s).
2. Description of the Prior Art
Traffic engineers typically collect data concerning traffic speed and density, vehicle size, loading and type, and vehicle condition as an aid in determining the design parameters for roads, highways, bridges and other structures. However, for multi-lane highways, acquiring the data required for complete evaluation and planning of these structures becomes quite difficult because of the need to monitor many lanes simultaneously. Indeed, the volume and complexity of the data required to make a complete evaluation of multi-lane roadways renders manual traffic counting impractical. As a result, automatic traffic recorders have been developed for recording data in a form which may be readily tabulated and evaluated.
Due to their electromechanical characteristics, piezoelectric materials such as piezoelectric polymer cables and films have been used as traffic sensors for acquiring traffic data. In the standard configuration, one piezoelectric sensor is disposed in each lane of traffic so that discrete electrical signals may be detected from each of the piezoelectric sensors. Unfortunately, while this technique works well for two-lane roadways, it becomes quite burdensome when traffic is to be monitored for more than two lanes of traffic. One major problem with such sensors is that they are difficult to install in the roadway and such installation requires substantial labor and creates major safety concerns. As a result, it is desired that as few easy to install sensors as possible be used to obtain the desired traffic data.
Such a standard traffic sensor is described by Myers in U.S. Pat. No. 3,911,390. Myers obtains traffic information by placing an elongated traffic sensor strip having a plurality of detector segments appropriately spaced along the sensor across a multi-lane roadway to monitor traffic in the lanes of the multi-lane roadway. The detector segments may each include a pair of parallel spaced conducting plates which generate an output signal when pushed together by the weight of a vehicle, or alternatively, the detector segments may each comprise a coaxial cable in place of the parallel spaced conducting plates. Generally, a separate detector segment is placed in each lane so that the lane may be discriminated; however, in an alternative embodiment, two or more coaxial cables are placed across the roadway to provide lane segregation. In the latter embodiment, the first coaxial cable extends completely across two lanes of traffic while the second coaxial cable extends only across one lane. The lane through which a vehicle passes is then discriminated by logically ANDing the positive outputs from each cable which are generated when the coaxial cables are deflected by the wheels of a vehicle. In this manner, the lane is discriminated in accordance with whether a positive pulse is received from just one or both cables.
The traffic sensor described by Myers typically has a low profile so that it is not readily visible by the motorists and has a gradually tapering profile so that it provides a smooth tire transition for a vehicle. The traffic sensor described by Myers is generally designed to be quite durable so that it can resist wear and damage from dirt or moisture. However, the durability of the sensor is improved by anchoring it in the roadway so that it will remain in position over a long period of time. Unfortunately, the sensors of Myers are difficult to install in the roadway, require the roadway to be closed for installation, and do not alleviate the above-mentioned safety concerns.
Traffic sensors have also been used to measure the dynamic loads exerted on a highway by traffic. For example, Siffert et al. describe in U.S. Pat. No. 4,712,423 a process for allegedly measuring the dynamic load exerted on a highway by the axles of vehicles by using the outputs of two piezoelectric cables installed in the roadway which are sensitive to the pressure and speed of vehicles passing thereover. In particular, the electrical pulses generated by the passage of vehicles over the sensors described by Siffert et al. are processed to extract weight information and speed information therefrom which is in turn used to calculate the dynamic load. However, such weigh-in-motion techniques, though relatively simple in theory, have proven difficult to implement in practice. Moreover, Siffert et al. do not disclose how to discriminate such information for different lanes of multi-lane roadways.
Similarly, Gebert et al. describe in U.S. Pat. No. 5,008,666 traffic measurement equipment including a pair of coaxial cables having piezoelectric materials and a vehicle presence detector embedded therein for detecting vehicle count, vehicle length, vehicle time of arrival, vehicle speed, the number of axles per vehicle, axle distance per vehicle, vehicle gap, headway and axle weights, and the like. This is accomplished by extending the coaxial cables including the piezoelectric materials across the roadway, measuring signals induced in the cable by passage of vehicle wheels thereover, and processing the signals to compute a total integrated spectral power of the measured signals so as to establish an empirical relationship between speed and weight of the vehicle wheels passing over the coaxial cables. However, as with Siffert et al., Gebert et al. install a separate detector in each lane and thus provide no means for collecting traffic data from multiple lanes using a minimum number of easy to install detectors.
It is desired to extend the traffic measurement techniques described by Myers, Siffert et al. and Gebert et al. to further include means for distinguishing traffic data collected from multiple lanes of a roadway using a minimum number of easy to install detectors. In particular, it is desired to develop a piezoelectric material which can generate pulses of different polarities or states in different longitudinal sections thereof so that, for example, if the piezoelectric material is extended across a multi-lane roadway, pulses of different polarities are generated in different lanes so as to uniquely identify those lanes. It is also desirable that the resulting traffic sensor be easy to install so that it can be placed across multi-lane roadways with minimum disruption of traffic. The present invention has been designed to meet these needs.